1. Field of the Invention
The present invention relates to a developing unit for an electrophotographic image forming apparatus. More particularly, the present invention relates to a developing unit for an electrophotographic image forming apparatus, which makes the rotation of a developing roller gear formed as a single body in a developing roller uniform, thereby improving an image quality.
2. Description of the Related Art
In general, image forming apparatuses form an electrostatic latent image corresponding to a desired image by radiating light on a photosensitive body, supply a developing agent to the electrostatic latent image, develop the electrostatic latent image with the developing agent, and print the developed image on a sheet of paper through both transfer and fusing operations.
FIG. 1 schematically illustrates a gearing train of a conventional developing unit for an electrophotographic image forming apparatus. Referring to FIG. 1, the conventional developing unit 20 for an electrophotographic image forming apparatus is rotatably installed in a photosensitive unit 10 on which a photosensitive drum is mounted, centering on a hinge 30. Here, the developing unit 20 includes a developing roller which faces the photosensitive drum, is rotated, and develops an electrostatic latent image formed on the photosensitive drum, a developing agent supplying roller which supplies a developing agent to the developing roller, and a developing agent agitating unit which agitates the developing agent. A photosensitive drum gear 11, a developing roller gear 21, a developing agent supplying gear 22, and a developing agent agitating gear 23 are formed as a single body in the photosensitive drum, the developing roller, the developing agent supplying roller, and the developing agent agitating unit, respectively.
In the above structure, if a printing operation is performed, a driving force is transferred from a driving unit disposed in a main body of an image forming apparatus to the photosensitive drum gear 11, and the driving force is transferred to the developing roller gear 21 through the photosensitive drum gear 11. The driving force transferred to the developing roller gear 21 passes idle gears 24, 25, and 26 sequentially and causes the developing agent supplying gear 22 and the developing agent agitating gear 23 to rotate.
In this procedure, in order to prevent a variation in an axis-to-axis distance between the photosensitive drum and the developing roller caused by the driving force, a force 41 is applied to the developing unit 20. Specifically, as shown in FIG. 2, a repulsive force F caused by the driving force is generated between the photosensitive drum gear 11 and the developing roller gear 21 in a direction of a pressure angle β of a gear. As shown in FIG. 3, the axis-to-axis distance between the photosensitive drum and the developing roller repeatedly varies by a perpendicular force component Fx of the repulsive force F in a segment where the center of the hinge 30 and a rotating portion of the developing roller gear 21 are connected to each other. Thus, in order to prevent the variation in the axis-to-axis distance between the photosensitive drum and the developing roller caused by the driving force, a force 41 is applied to the developing unit 20. Fy of FIG. 2 denotes a force component in a direction of a segment where the center of the hinge 30 and the rotating portion of the developing roller gear 21 are connected to each other. Reference numeral 21′ of FIG. 3 denotes a developing roller gear of which position varies by the driving force.
However, even though the force 41 is applied to the developing unit 20 as described above, as shown in FIG. 4, the axis-to-axis distance between a photosensitive drum 51 and a developing roller 61 repeatedly and finely varies by the repulsive force F caused by the driving force. The fine variation in the axis-to-axis distance between the photosensitive drum 51 and the developing roller 61 causes overlapping and separation between a pitch circle 12 of the photosensitive drum gear 11 and a pitch circle 22 of the developing roller gear 21 together with a variation in the amount of a nip between the photosensitive drum 51 and the developing roller 61 (see FIG. 5). Reference numeral 61′ of FIG. 4 denotes a developing roller of which position finely varies by the driving force. Reference numeral 22′ of FIG. 5 denotes a pitch circle of the developing roller gear 21 overlapping with the pitch circle 12 of the photosensitive drum gear 11, and reference numeral 22″ of FIG. 5 denotes a pitch circle of the developing roller gear 21 spaced apart from the pitch circle 12 of the photosensitive drum gear 11. If the axis-to-axis distance between the photosensitive drum 51 and the developing roller 61 repeatedly varies, contact between the pitch circle 12 of the photosensitive drum gear 11 and the pitch circle 22 of the developing roller gear 21 becomes nonuniform. The nonuniform contact between the pitch circles 12 and 22 disadvantageously causes jitter in an image.